1. Technical Field
The present invention relates to a wind turbine generator and a method of controlling the wind turbine generator, and more particularly, to the yaw control of a wind turbine generator.
2. Description of the Related Art/Background Art
One important control for improving the efficiency of a wind turbine generator is yaw control for controlling the direction of the wind turbine rotor so that the direction of the wind turbine rotor corresponds to the wind direction. When the wind turbine rotor faces into the wind, the wind turbine generator achieves maximum efficiency. Accordingly, the direction of the wind turbine rotor needs to be controlled by performing yaw rotation of a nacelle, in which the wind turbine rotor is mounted, in accordance with a wind direction. There have been various challenges for a yaw rotating mechanism or a yaw control technique. For example, Japanese Patent Application Laid-Open No. 2004-285858 discloses a technique that detects the wind direction and wind power by a laser-type wind vane/anemometer and performs yaw control on the basis of the detected wind direction and wind speed. Further, Japanese Patent Application Laid-Open No. 2005-113899 discloses the configuration of a drive mechanism for performing the yaw rotation of the nacelle.
One of the important aspects of yaw control for a wind turbine generator is to reduce the number of yaw rotations of the nacelle. The nacelle is heavy and, for this reason, if the number of yaw rotations of the nacelle is large, the mechanical load applied to the rotating mechanism for rotating the nacelle or the braking mechanism for stopping nacelle rotation increases. As a result, mechanical wear on these mechanisms increases. It is preferable that the number of yaw rotations be minimized in order to reduce wear on the rotating mechanism or the braking mechanism.
The control logic used to meet these demands for the most common yaw control is a control logic performing yaw rotation of the nacelle so that deviation from the wind direction becomes zero (that is, the orientation of the wind turbine corresponds to the newest wind direction) when a state where the absolute value of the deviation (the wind direction deviation) between the orientation of a wind turbine (that is, the direction of a wind turbine rotor) and the actual wind direction is larger than a predetermined threshold continues for a predetermined duration (for example, 20 seconds) as shown in FIG. 18. Unless the absolute value of the wind direction deviation exceeds a threshold, yaw rotation is not performed in this control logic. Accordingly, it may be possible to reduce the number of yaw rotations by appropriately setting the threshold.
As shown in FIG. 19, one problem with this control logic is that, in a situation where the wind direction changes gradually over a long time (typically, several hours), the magnitude of the wind direction deviation is not reduced on average. In general, the wind at a certain location has high degree of turbulence during the day and the wind direction changes at random, but it is often the case that at night the wind direction does not randomly change. In other words, the wind situation often changes at night so that the wind direction changes over an extended period of time. According to the above-mentioned control logic, the magnitude in the wind direction deviation approaches zero on average in situations where the degree of turbulence is high and the wind direction changes at random. However, if the wind direction changes gradually over a long time (typically, several hours) as shown in FIG. 19 (A of FIG. 19), the wind direction deviation becomes zero only momentarily (C of FIG. 19) in the above-mentioned control logic even though the yaw rotation is repeated (B of FIG. 19). Accordingly, the average magnitude of the wind direction deviation is not reduced. This is not preferable for improving the efficiency of the wind turbine generator.